In recent years, according to an increase in speed of networks and rapid spread of personal computers and digital television receivers to general homes, various multimedia services are widely put to practical use. In particular, digital broadcast systems, content delivery systems using the Internet, and the like, it is a general practice to adopt a form of compression-encoding content data such as video and sound on the basis of standards such as the MPEG (Moving Picture Experts Group) system, setting a fraction of encoded data, which is the compression-encoded content data, as a packet, and transmitting a set of packets as a stream signal, or recording the set of packets in a hard disk, a DVD, a memory card, or the like. An information apparatus such as a personal computer, a set-top box, a portable information terminal apparatus, or a cellular phone, which receives such a stream signal, extracts the encoded data included in the stream signal and applies decoding to the extracted encoded data to restore the content data.
As such a system for compression-encoding an image, there is the JPEG (Joint Photographic Experts Group) system for compression-encoding a still image, which is widely used for a digital camera and the like. As a system for compression-encoding a moving image, there is the MPEG-1 system for recording the moving image in a CD-ROM or the like. Consequently, a basic technique such as an image compression encoding method based on, for example, discrete cosine transform is established. On the basis of such a basic technique, for example, the MPEG-2 system used in digital broadcasts, the MPEG-4 system used for contents provided by using the Internet, and the MPEG-4AVC system for the purpose of high compression encoding are used as standard systems in a field of treatment of image signals.
A basic algorithm for such image compression encoding is a hybrid compression encoding system in which the discrete cosine transform system, which is one of orthogonal transform systems, is combined with a motion compensation prediction system. In the hybrid compression encoding system, image compression encoding is applied to an image by combining both data obtained by discrete cosine transforming data itself in one picture and data obtained by discrete cosine transforming motion compensation inter-picture prediction data.
In such image compression encoding, the discrete cosine transform is performed in a block unit including a predetermined number of pixels. Respective pixel data forming this block are transformed into plural coefficient values indicating amplitude intensity for each of frequencies from a direct current component to a high-frequency component by the discrete cosine transform. Such respective coefficient values are first quantized by a method based on each compression encoding system to be transformed into quantized coefficient values. Further, the respective quantized coefficient values are rearranged according to a scan pattern indicating order of the respective data in order to improve a compression ratio by variable length encoding. Thereafter, the rearranged respective quantized coefficient values are sequentially subjected to variable length encoding according to the order of the scan pattern.
As such variable length encoding, run level encoding for allocating a unique code to a set of a run, which is the number of preceding zero quantized coefficient values, and a level, which is a non-zero quantized coefficient value, to realize compression of data is used. Compression encoding of an image is performed on the basis of such an algorithm and encoded data including the coefficient data compressed as explained above is generated.
An original image can be restored by applying decoding in association with the compression encoding to the encoded data generated by the compression encoding. In other words, the quantized coefficient values conforming to the order of the scan pattern are restored from the data of the run and the level included in the encoded data. Further, the quantized coefficient values are rearranged according to scan pattern data for the scan pattern in the compression encoding. Consequently, the respective quantized coefficient values arranged in predetermined positions on the block are restored. Further, inverse quantization is applied to these quantized coefficient values and inverse discrete cosine transform is applied to the respective coefficient value restored by the inverse quantization, whereby pixel data in the block unit is restored.
In the MPEG-4 system and the like, in addition to these kinds of encoding, a technique called DC/AC prediction encoding is adopted as a technique for reducing a generated code amount of a block for which intra-picture prediction is performed.
FIG. 9 is a conceptual diagram for explaining the DC/AC prediction encoding. As shown in FIG. 9, in the DC/AC prediction encoding, a DC coefficient as a direct current component of an encoding target block and an AC coefficient in a first row or a first column as an alternating current component are predicted on the basis of a block adjacent above the encoding target block (an upper adjacent block), a block adjacent on the left of the encoding target block (a left adjacent block), or both the upper adjacent block and the left adjacent block. Only a prediction error of the prediction is encoded to hold down an encoded data amount. Besides the MPEG-4 system, such a technique of DC/AC prediction encoding is used in plural encoding systems such as the H.263 system.
Various devices for applying DC/AC prediction decoding to encoded data subjected to such DC/AC prediction encoding have been proposed. For example, in Patent Document 1, a hardware device for performing DC/AC prediction processing for the MPEG-4 system is proposed.
However, in the case of a method of configuring a decoding circuit individually for each of various image encoding systems in association with the image encoding system as the conventional decoding circuit, it is necessary to design a DC/AC prediction circuit individually for the respective image encoding systems. Therefore, there are problems in that, for example, a circuit size is increased and, for example, when a new image encoding system is proposed, it is difficult to quickly cope with the image encoding system and redesign of an LSI (large scale integrated circuit) for performing decoding is necessary.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-103472